The invention involves an axle-drive unit for motor vehicles having a first and a second driven axle which contains a first and a second differential in a housing driven by an engine/transmission block, the first differential dividing the torque fed to it between a first half axle of the first driven axle and the second differential, and the latter furthermore dividing the torque fed to it between a second half axle of the first driven axle and a power take-off for the second driven axle, the two differentials being spur-gear-type planetary gears with parallel axes, the sun wheels of which are in each case connected in terms of drive to the half axles of the first driven axle, and planet wheels of the two differentials meshing with their common ring gear, the one planet carrier being connected in a rotationally fixed manner to the housing and the other planet carrier being connected in terms of drive to the power take-off for the second driven axle.
An axle-drive unit of this type is disclosed in AT 405 923 B. In the latter, owing to the particular design and arrangement of the two differentials, optimum adaptation of the moment distribution ratio is achieved with a minimum outlay on construction. Provided between the driven housing, which contains the two differentials, and the power take-off for the second driven axle is a fluid friction clutch as a longitudinal differential lock. The latter is not only extremely bulky, it also has the disadvantage of acting only as a lock for the differential between the two axles. Locking of the other differential, the differential between the two wheels of the first driven axle, is not possible.
It is thus the object of the invention to achieve an at least limited locking both of the axle differential and of the longitudinal differential with a minimum outlay on construction.
According to the invention, this is achieved in that
a) the housing has a machined inner surface which surrounds the ring gear with little clearance,
b) the ring gear is so thin in the radial direction that it is deformed in a lobe-like manner by the radial component of the tooth forces exerted by the planet wheels,
c) its outer circumferential surface thereby being placed against the inner surface of the housing in a manner producing friction at least locally, as a result of which a braking moment acts on the ring gear.
Locking thereby takes place without additional components between the housing and ring gear. Moreover: the lock acts both on the axle differential of the first driven axle and on the interaxle differential between the two driven axles. The locking behavior also complies with the requirements: the action occurs only at high torques, i.e. when it is actually required, but not in towing mode or during gentle cornering. Since the tooth forces between the planet wheels and ring gear are approximately identical in both differentials, the bending stresses over the axial length of the ring gear are also approximately the same. In addition, in the locked state, the housing exerts a supporting action on the ring gear limiting the local expansion, which prevents excessive deformation of the ring gear and tooth fractures. This is all of benefit to the service life of the ring gear.
In a preferred embodiment, the driven housing is divided in a radial plane, that part of the housing which forms the inner surface rises above the radial plane of separation and an undercut is provided between this part and the radial surface. This undercut is used for decoupling screwing stresses caused by the bolts acting on the outside of the housing parts and thermal stresses caused by heating of the inner surface in the locking mode. That part of the housing which forms the inner surface thereby remains dimensionally accurate. A further contribution to this resides in the housing part with the inner surface having radial cooling ribs on its outside. This enlarges the area for the transfer of heat to the oil or to the surrounding transmission atmosphere.
There is great design freedom for the design of the frictional surfaces—both of the inner surface of the housing and of the outer circumferential surface of the ring gear. Machined, raised zones of limited axial width have proven advantageous. The reduction in the contact area may result in a reduction in the required contact pressure for a certain braking action and makes it easier to keep to the functionally desired, exacting tolerances.